How Do Chicks Raised By Parents of Another Species Know Who They are?

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An advanced tracking system revealed the secret habits of young cowbirds, like this one with a radio transmitter on its back. Credit: Mike Ward

Birds like cuckoos outsource parental responsibilities. They surreptitiously lay their eggs in the nests of other species that raise these interlopers along with their own. Parent birds invest tremendous effort in raising their chicks. Foisting child care duties on others, called brood parasitism, frees these parasitic parent birds to live their own lives. Despite the obvious advantages, only one percent of all birds force other species to provide free parental care.

However, it’s not all hunky-dory for the changelings. Chicks raised by parents of another species could face an identity crisis. If their foster parents and siblings belong to a different species, how do the free-loading chicks know who they are? Will they learn to sing the song of the adopted family or their own?

Chicks learn songs, diet, social etiquette, and migratory routes from their parents. These are not inborn nor can they be learned from foster parents. The circumstances of their birth pose a different set of problems to these youngsters.

If a crow-raised cuckoo chooses a crow for its mate, assuming the crow consents, such unions may not produce offspring, an evolutionary dead end. Or they could produce hybrids, but these are virtually unknown among the hundred-odd brood parasites. When cuckoos become adults, they seek cuckoo mates instead of crows, demonstrating they know who they are.

Biologists from the University of Illinois at Urbana-Champaign and University of New York, U.S., have found the answer to a small part of the puzzle.

The brown-headed cowbird, a fruit and insect-eating bird of North America, lays its eggs in the nests of others, like prothonotary warblers. These species have different habits. Adult cowbirds live in open fields and prairies, while warblers live in forests. Cowbirds live mainly off seeds although they eat insects too, but warblers eat insects and caterpillars. Young cowbirds somehow dissociate themselves from their foster parents and do what cowbirds do.

Could biological cowbird mothers play a role in grooming their offspring? This is not a far-fetched idea. Cowbird mothers don’t just drop their eggs in warbler nests and abandon them to their fate. They keep an eye on those nests. If the foster parents shove the strange eggs overboard, cowbird mothers vandalise the nests, forcing the hosts to build another nest. When their young beg for food, they also call in response. Another study found 36% of adult and young cowbirds that were trapped together were closely related.

At the beginning of the study, Matthew Louder, the lead author, and his team speculated cowbird mothers, hanging around the nests, may come to the aid of their offspring during fledging time. Even if parents didn’t actively encourage their fledglings to follow them, perhaps the youngsters became fixated on their mothers whom they may see often around the nest and follow them. To test this, the researchers fitted radio transmitters to 15 fledgling cowbirds and 15 adult female cowbirds found near the nests.

Tracking these birds was not the same as tracking other animals. “I could only manually radio-track one individual at a time,” says Louder. “I tried the exhausting approach of following radio-tracked juvenile cowbirds as much as possible, watching a single bird from morning ‘til sunset. If the bird flew far, sometimes a kilometre, I would run through the forest chasing it. But the biggest problem is that I was going home to rest in the evenings. Like a naive parent, I assumed that these teenagers stayed put all night.”

Professor Michael Ward, University of Illinois and a co-author of the study, came up with another approach. He constructed an automated system consisting of three radio towers, each with six antennas, which provide 360º coverage. The system tracked one bird at a time every one to two minutes.

The mavericks

The team put up three radio towers, each with six antennas on it, to track the cowbird juveniles and the female cowbirds day and night. Credit: Matthew McKim Louder​

To their surprise, they found the youngsters flew alone from the forest, without the supervision of adult female cowbirds. Although some female cowbirds hung around the forest at fledging time, they were not there when the fledglings wanted to leave. Neither were the offspring within the home range of their mothers. Cowbird mothers didn’t play a role in helping their young fledge as they had thought.

Louder thought something was wrong with the data. “Fortunately, there were still two juveniles that I could manually follow at sunset. I expected that they may have been roosting on the ground, and it was messing with the automated telemetry system. Much to my surprise, I watched each bird fly out of the forest and out into a field. I would then race over to the grassy field and find them. I followed each bird over a series of nights, and they both flew out of the forest at night alone, roosted in the field, and returned to beg from the little warbler parents the following morning.”

The researchers speculate the juveniles may have an innate preference for specific roosting habitats. Roosting alone at night may help young cowbirds become independent of their foster parents.

“This is what makes this behaviour so peculiar,” says Louder. “The birds not only leave at night, which should be dangerous for depredation from owls, they go to places they have never been before. They are potentially leaving to a place more dangerous than staying in the forest. This is one reason that led me to believe this behaviour is important, otherwise it would not persist.”

Chicks aren’t fed at night, so it’s a good time to venture out on their own. But how do they learn what cowbirds do?

Adult cowbirds fly to communal roosts in groups every evening. By making these solitary forays, juveniles stand a greater chance of meeting up with them. The research team can’t explain how the fledglings identify their own kind and find their way into a cowbird flock. But they observed several females with juveniles at a communal roost, so the young birds are clearly able to make that switch from foster to biological families.

“Clearly, there’s a lot more to these birds than people would have thought,” said Jeff Hoover, avian ecologist, Illinois Natural History Survey, in a press release. “We still have more layers to peel away from this onion that is the cowbird.”

Cracking the mystery involves a technological challenge. “Our radio transmitters would need to last longer,” says Louder. “The little backpacks are small, 0.9 grams, and do not hinder juvenile survival. But this means that the battery life only permitted me to follow birds for approximately 30 days. The few birds that I found out in the fields after independence from their warbler foster parents were usually alone. So, they might take another week to find a group of cowbirds and recognize their own kind. To really get at this question, I would need satellite transmitters that are currently too heavy. Or, we would have to put up more radio towers to track the birds while out in the fields and at a communal roost.”

There seems to be a blurred line between what a fledgling knows innately, such as where to roost, and what it has to learn from its biological parents, like what to eat and how to eat.

“This is why I find the cowbird and other brood parasites, like cuckoos, so fascinating,” says Louder. “Our study provides a small piece to the puzzle of how juvenile cowbirds manage to avoid imprinting on foster parents, yet remember to parasitise them as adults.”

While getting others to take care of one’s young appears to be a lazy way of parenting, the chicks have to brave predators and work harder by flying back and forth to avoid becoming fixated on their adopted parentage. At least the chicks can look forward to an adulthood with few child-care responsibilities.